1. Field of the Invention
This invention relates to a gas discharge image display such as a large size color display or an electronic bulletin board using a number of gas discharge lamps to provide a large screen.
2. Description of the Related Art
The present applicant invented a display in which pairs of planar electrodes are located on the outer wall of a dielectric container such as a glass bulb and a number of fluorescent lamps within which a rare gas such as xenon is sealed are disposed, whereby a voltage applied to the planar electrodes is controlled for controlling a discharge and light emission of the fluorescent lamps for partially displaying an image. The display is shown in Japanese Patent Laid-Open No. Hei 5-82101, for example. Displays of this type provide high intensity and high efficiency because an excimer of a rare gas is generated by a discharge and fluorescent material is excited to emit light by ultraviolet rays radiating from the excimer.
FIGS. 1A and 1B are a perspective view and a sectional view showing a fluorescent lamp used to form a display of this type shown in Japanese Patent Laid-Open No. Hei 5-82101, for example. In the figures, numeral 1 is a fluorescent lamp, numeral 2 is a glass bulb forming the fluorescent lamp 1, numeral 3 is a fluorescent layer formed substantially on the half face of the inner wall of the glass bulb 2, and numeral 4 is a light output section, opposite to the fluorescent layer 3, where no fluorescent layer is formed. Numerals 5a and 5b are external electrodes, located on the outer wall of the portion in which the fluorescent layer 3 is formed, for making up a picture element 6. A number of the electrode pairs are disposed in the axial direction of the glass bulb 2. Numeral 7 is a recess formed by recessing the glass bulb 2 between picture elements. A rare gas such as xenon is sealed within the glass bulb 2. In FIG. 2, numeral 8 is a display where a plurality of the fluorescent lamps 1 are disposed and the electrodes of the picture elements are connected like a matrix.
When an alternating voltage is applied from the external electrodes 5a and 5b, a discharge occurs between the electrodes, whereby an excimer of a rare gas occurs on the surface of the electrode section on the inner wall of the glass bulb 2. The fluorescent layer 3 formed on the inner wall of the glass bulb 2 is excited by ultraviolet rays radiating from the excimer, and visible light is emitted from the light output section 4. Since only the fluorescent material in the portion corresponding to the electrode pair causing the discharge to occur emits light at this time, the electrode pair can be used as a picture element. Therefore, an image can be displayed by disposing a number of the fluorescent lamps.
On the other hand, an AC plasma display panel (AC-PDP) is well known as a display where power applied from external electrodes is supplied via a glass, a dielectric to the inside of discharge space and discharge light emission occurs, thereby displaying an image.
One of the drive systems of the AC-PDP is a memory drive. The AC-PDP has a memory function in which the light emission panel itself can easily continue two states of discharge light emission and off. The drive system using the memory function is a memory drive. The operation period of the memory drive is divided into write, support, and erase. A picture element causing a discharge once in the write period continues discharge light emission at a lower voltage than the discharge start voltage during the support period, and stops discharge light emission in the erase period. Thus, unlike other drive systems such as refresh drive in which light is emitted only when scanning, the memory drive system can display an image at high intensity.
FIGS. 3A and 3B are a perspective view and a sectional view showing the structure of a conventional AC-PDP described in Ken'ichi OOWAKI and associates "Plasma Display" Kyoritsu Shuppan, 1983, pp. 21-22, for example. In the figures, numeral 8 is a conventional AC-PDP and numerals 2a and 2b are glass plates forming the conventional AC-PDP 8. On the inner surfaces of the glass plates 2a and 2b, linear electrodes 5a and 5b are located crossing at right angles with dielectric layers 11 and a discharge space 13 between. Grid points of the linear electrodes 5a and 5b become picture elements 6 for emitting light by a discharge. On the inner surfaces of the glass plates 2a and 2b, dielectric layers 11 are formed covering the linear electrodes 5a and 5b, and further a protective layer 12 is formed on each of the dielectric layers 11. Fluorescent materials (not shown) for emitting red (R) light, green (G) light, and blue (B) light are formed at proper points inside the AC-PDP 8 by a method such as printing. A mixed gas of helium and xenon is sealed within the AC-PDP.
An alternating voltage less than the discharge start voltage is always applied between linear electrodes 5a and 5b of the AC-PDP (support pulse). When a voltage exceeding the discharge start voltage, a write pulse, is applied between electrodes, a discharge is started between the electrodes. After this charges accumulate on the dielectric layer surface inside the AC-PDP to form barrier charges, thus discharge light emission is continued even with a support pulse of a voltage less than the discharge start voltage. Next, when a voltage pulse (erase pulse voltage) is applied so as to cause a faint discharge between electrodes, space charges generated by the discharge are recombined with the barrier charges on the dielectric layer surface to eliminate the barrier discharges. Therefore, after this, no discharge light emission occurs even if the support pulse voltage is applied.
FIGS. 4A and 4B are drawings showing an erase technique (broad erase method) and its erasable range (erase characteristic) of the conventional AC-PDP described in the document mentioned above, for example. In the figure, support pulse SP is applied between linear electrodes 5a and 5b of the conventional AC-PDP 8 to continue discharge light emission, and erase pulse EP causes a faint discharge to occur for stopping the discharge light emission. The erase pulse has substantially the same width as the support pulse and has a smaller voltage value than the support pulse. FIG. 4B shows the relationship between erase pulse voltage values (horizontal axis) and support pulse voltage values (vertical axis), wherein the hatched portion 14 is the erasable range in which the support and erase pulse voltage values are set.
With the AC-PDP, a narrow erase method described in the document mentioned above is available in addition to the broad erase method, whereby an erase pulse having substantially the same voltage value as a support pulse and having the short application time is applied for erasing. The narrow erase method provides a large erasable range compared with the broad erase method. When an erase pulse is applied and a discharge occurs, voltage is removed before a barrier charge of opposite polarity is formed. Thus, the barrier charge remaining just after the voltage is removed sucks in a space charge generated by a discharge by Coulomb force, combines with it, and disappears. Since the broad erase method performs forced suction by applying external voltage for recombining the space and barrier charges with each other, the erasable range forms substantially a triangle. In contrast, since the narrow erase method recombines them by a natural suction force of the barrier charge itself, the barrier charge always converges to zero, thereby enlarging the erasable range.
Although it is an effective means to use the memory drive system already established with the AC-PDP for driving the gas discharge display by excimer light emission described above, the following problems arise:
First, the gas discharge display where a number of fluorescent lamps using excimer light emission are disposed and the electrodes of picture elements are connected like a matrix as described above differs from the AC-PDP greatly in picture element size, and thus differs in discharge characteristic. Therefore, even if the erase technique of the AC-PDP is adopted as it is to use the memory drive system for drive control, space charges remain in large amounts in a large discharge space and an erase operation is difficult to perform.
Next, fluorescent lamps using fluorescent materials of different luminous colors differ in electric characteristics such as the discharge start voltage and minimum support voltage depending on the type of fluorescent material of the fluorescent layer formed on the electrode section surface. Therefore, even if an attempt is made to perform memory drive at an image display where fluorescent lamps of different luminous colors are located, the voltage to be applied varies from one color to another, thus sufficient control is not provided from the simple connection of the electrodes in a matrix form. Particularly at erasing, the erasable range for one color slightly overlaps with that for another color, and control cannot be performed.